DEAR DR. DONOHUE: I just graduated from high school and am off to college this fall. I want to study sports medicine. I am interested in everything that has to do with athletics and am fascinated by the science behind it.

I have read about the body’s core, but I don’t understand its significance or exactly what it is. What is it? How do you train it, and why? – R.B.

ANSWER: The body’s “core” is a group of around 29 muscles in the middle part of the body. Those muscles stabilize the spine and transfer power from the lower half of the body to the upper half – from the legs to the arms and shoulders.

The abdominal muscles, spinal muscles, hip muscles and pelvic muscles are core muscles.

Without strong core muscles, the spine couldn’t stay upright when a person takes a step.

Core muscles are most important in twisting motions. Strong core muscles prevent back problems, and for that reason alone they’re most important.They also figure into almost every athletic movement. Throwing a baseball is a good example. The power of a throw doesn’t come only from the shoulders and arms. It comes from the leg, hip, abdomen and back muscles.

Core muscles transfer the power generated in the lower body to the shoulders and arms. The same muscles are needed to swing a golf club, row a boat and hit a tennis ball.

There’s no single exercise for core muscle training. So many muscles are involved that they require a variety of exercises. A very simple core-strengthening exercise is standing on one leg.

A more advanced core exercise is a modification of the push-up.

In the push-up position, when the body is off the floor with arms extended and weight supported by the palms and the feet, twist to the right and raise the right arm off the floor and straight up.

The body weight is now supported by the palm of the left hand and the feet.

Return to the starting position and make the twist to the left. Do as many twists as you can, rest and repeat. This is a difficult exercise, and only those in excellent condition should attempt it.

DEAR DR. DONOHUE: Where does jumping rope stack up as an exercise? I like to do it. Is it hard on the knees?

My knees don’t bother me, but I’ve been told that they will if I persist. How many calories does it burn? – R.R.

ANSWER: Jumping rope stacks high on the exercise scale. Boxers wouldn’t do it if it wasn’t a great conditioner.

It not only provides aerobic benefits – benefits for the heart and blood pressure – but it improves agility, coordination, balance and speed.

Jumping at a moderate speed burns around 10 calories a minute. That’s proof of the energy requirement that it takes to skip rope.

If you don’t jump exceedingly high, then it’s not hard on the knees. You need to lift the feet only an inch or two off the ground.

And if you choose a surface that gives a bit, that’s additional knee protection. A lawn is a good place to jump.

The length of the rope should be such that when standing on the rope’s center, you can reach the ends of the ropes to just under your shoulders.

A jumping speed of 70 turns a minute is a good rate. Most people will find that rate exhausting at first.

Jump for a minute or two, rest, resume jumping and keep alternating with rests. As you become accustomed to it, take fewer and shorter rests.

DEAR DR. DONOHUE: After 18 holes of golf, the tendons on the back of my heels hurt. What should I do? – C.K.

ANSWER: The likely cause is inflammation of the Achilles tendon, the tendon of the calf muscle. It attaches to the heel.

Don’t play golf for two weeks. Take Aleve if there’s no reason for you not to use it. Put warm compresses on it for 15 minutes three times a day.

Try heel inserts, obtainable in every drugstore. When you start playing again, ice the tendon for 10 to 15 minutes after you’re through playing.

DEAR DR. DONOHUE: My 16-year-old daughter has had periods for two years. She’s had menstrual cramps ever since she started.

They don’t keep her from being active, but they do make her somewhat miserable. What over-the-counter medicine can she safely use? – C.N.

ANSWER: Menstrual cramps come from the contraction of uterine muscles.

Those contractions are caused by the release of prostaglandins, hormonelike substances that are at peak production during the menstrual period. Medicines like Advil or Aleve are good anti-prostaglandin medicines.

If your daughter’s cramps continue, you ought to involve the family doctor, who can find out if this is simply a normal variant or something that requires more attention.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com